Replicated aluminium foam | EPFL Graph Search

This thesis explores the processing and properties of microcellular aluminium-based materials produced by replication. It includes the development of a pressure infiltration process for net-shape component production, structural characterization of the foams, measurement of their fluid flow permeability and their uniaxial deformation characteristics, together with contributions towards explaining and modeling these properties. Sodium chloride particles are classified and compacted to form an open-pore pattern that is infiltrated with molten pure aluminium under gas pressure. After solidification of the metal, the salt is dissolved in water, yielding an open-cell aluminium foam, the structure of which replicates in negative that of the preform. Resulting highly porous aluminium foams (or "sponges"), regular and homogeneous in pore size, pore shape and density, are produced with an average pore size ranging from 5 to 500 μm and a relative density between 10 and 35 % (corresponding to porosity between 65 and 90 %). The structure of these replicated foams is characterized by optical and scanning electron microscopy. A collaboration with Ms A. Marmottant and Dr. Luc Salvo, GPM2, INP Grenoble, France, allowed three-dimensional images to be produced by X-ray tomography using synchrotron radiation, at the ESRF in Grenoble, France. The Darcian fluid flow permeability of replicated foams is measured varying the average cell size (75 and 400 μm) and the relative density (from 12 to 32 %). Data show the expected dependence on the square of the pore size and agree with other experimental data in the literature for coarser aluminium foams produced by a different casting process. A predictive model for the permeability of open-pore microcellular materials is derived based on a quantitative description of the foam structure, leading to a simple analytical expression. Predictions agree well with present and published experimental data. Mechanical tests are performed in both compression and tension to evaluate the response of pure aluminium replicated foams and its dependence on principal microstructural parameters. Present and published experimental data for the stiffness and the flow stress are compared with the usual Gibson-Ashby equations and with predictions of mean-field models for the deformation of composite materials adapted to metal foams. The best predictor of the behaviour of replicated foams is found with the differential effective medium calculation of composite elastic moduli. This, coupled with the modified secant modulus model of composite non-linear deformation by Suquet is also shown to provide a good predictor of the plastic flow characteristics of these foams. Damage accumulation during foam deformation is evidenced, lowering the foam stress in both tension and compression and causing an evolution in foam stiffness and electrical conductance. In-situ compression and tension tests are performed to observe by X-ray microtomography the mechanisms of deformation and fracture at the foam cell scale. The formation of plastic hinges in compression and strut necking and fracture in tension are evidenced. Damage is shown to govern the tensile failure of the foams, in accordance with theory. The influence of microstructural features on the mechanical behaviour of replicated aluminium foams is investigated. It is shown in particular that the average pore size of the foams influences their plastic flow stress and work hardening rate, both increasing with decreasing pore size. Variations in the heat treatment of the foams are used to show that this plasticity size effect is dislocational in nature, and that it has its origin in dislocation emission during thermal excursions of the Al/NaCl precursor of the foam, before dissolution of the salt. The infiltration pressure and the salt grain shape are also shown to influence the foam structure and the foam uniaxial compressive mechanical response.

Multiaxial Yield and Fracture of Replicated Microcellular Aluminium

Etienne Combaz

This work addresses the behaviour of replicated microcellular pure aluminium under multiaxial stress states and in the presence of stress and strain localization sites. Processing of the foam was conducted in-house, using the replication process. The main processing steps are: (i) infiltrating a bed of sieved sodium chloride (NaCl) particles with pure molten aluminium under Argon pressure, (ii) solidifying the casting directionally, (iii) machining samples in the solidified composite obtained, and (iv) dissolving the salt in water containing chromate corrosion inhibitors. The main advantage of this process is its great versatility; notably, it allows tailoring independently both the size of the final pores in the replicated foam (by sieving the initial particles), and the relative density of the obtained foam material (by compacting the salt particles before infiltration). The first part of this work is dedicated to the study of the multiaxial yield behaviour of replicated foams. For this purpose a new triaxial loading frame was built to test cubical specimens. In Chapter 3 preliminary tests on pipe specimens are presented together with the results obtained on the triaxial device for 75 µm pore size foam cubes under axisymmetric and biaxial stresses. These show that the yield surface depends on all three stress tensor invariants, a dependence that has been suggested recently by theory and finite element simulation, but hitherto not shown in microcellular metal. A simple empirical expression taking the influence of the third stress tensor invariant into account is proposed based on the data. Experiments on 400 µm pore size foams under axisymmetric and biaxial stress and in three different deviatoric II-planes (planes in stress space perpendicular to the hydrostatic axis) confirm the dependence of the yield surface on the third stress tensor invariant, the yield surface becoming trilobal at elevated hydrostatic stress (tensile or compressive) in the II-planes, while being circular at zero hydrostatic stress, a feature that is captured by the empirical expression proposed on the basis of the present experiments. The second part of this thesis addresses foam deformation and fracture in the presence of cracks, notches or holes. The fracture toughness of replicated 400 µm pore microcellular aluminium is explored using disc-shaped compact tension testing broadly along the lines of the ASTM E1820-08a standard. The material shows marked R-curve behaviour, and the presence of bridging ligaments in the crack wake. Fractography reveals that the crack propagates via the rupture of struts normal to the crack plane, this being accompanied by a degree of plastic deformation of the struts near the crack plane that is made visible by slip markings along the strut surface and that increases in extent with Vm. Measured crack initiation and steady state propagation J values vary roughly as Vm3; this is a stronger dependence than has been observed in commercial (closed-cell) metal foams. A simple model is proposed to describe the initiation toughness of ductile open-cell foams, based on an estimate of the crack tip opening displacement at the moment when a strut aligned in the loading direction fails by ductile rupture just ahead of the crack tip. The model agrees well with the data up to a relative density of 20%, accounting in particular for the observed scaling of the toughness of replicated foams on the third power of the relative density. Finally the tensile flow and failure of flat dog-bone specimens containing a cylindrical hole, and of cylindrical notched samples of microcellular aluminium is studied. Both 400 µm and 75 µm pore size foams exhibit a notch strengthening effect, i.e., the peak failure stress increases as the depth of notches in cylindrical samples increases. In dogbone samples, the presence of a hole ranging from 0 to 4mm in diameter (in a sample 9mm wide) does not affect the net section peak failure stress of the 75 µm foam while the 400 µm pore size foam exhibits a slight increase in net section failure stress as the hole diameter is increased to 2 mm. Plastic flow curves for notched and hole-containing samples are well predicted by a finite element simulation based on the Deshpand – Fleck flow law, showing that the observed trends in the data are predominantly mechanical in nature, and strongly linked with the presence of triaxiality at the center of the notched samples.

EPFL2010

Yield surface of polyurethane and aluminium replicated foam

Claudio Bacciarini, Raphaël Charvet, Etienne Combaz, Willy Dufour, Andreas Mortensen

It has been proposed in the literature, based on theoretical considerations and on finite-element calculations, that all three stress tensor invariants govern the yield surface of cellular materials. Recent experiments on 75 gm pore size aluminium replicated foams (Combaz E, Bacciarini C, Charvet R, Dufour W, Mortensen A. Multiaxial yield behaviour of AI replicated foam, submitted for publication) showed such a dependence of the yield surface in axisymmetric tests. This study explores the yield behaviour of 400 gm pore size aluminium replicated foams: experiments confirm the influence of the third invariant on the yield surface shape, together with the observations that (i) the yield surface shape does not depend on relative density and (ii) measured flow vectors conform with normality. A simple parabolic model fitting data in the previous study also captures well the present data under all tested stress states (biaxial, axisymmetric and Pi-planes in stress space). Biaxial and axisymmetric tests are also performed on 400 pm pore size polyurethane (PU) replicated foams with a similar mesostructure. Results show yield to occur at a value lower than predicted by micromechanical models for both matrix materials (aluminium and PU). This suggests that the "knock-down" factor usually observed between predicted and observed stress values probably cannot be explained by a lowered yield stress in the material making the foam. The data also suggest an influence of the matrix nature on the yield surface geometry. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Elsevier2010

Multiaxial Yield and Fracture of Replicated Microcellular Aluminium

Etienne Combaz

EPFL2010